1. Field of the Invention
The present invention relates to an image forming apparatus wherein an image formed on an image bearing member such as a photosensitive body, a dielectric body and the like is transferred onto a transfer material born by a transfer material bearing member, and more particularly it relates to a color image forming apparatus wherein a color image is formed by superposing plural color images on the same transfer material.
2. Related Background Art
Conventionally, various color image forming apparatuses have been proposed, and a typical full-color image forming apparatus of electrophotographic type is shown in FIG. 5. In FIG. 5, the color image forming apparatus has a cylindrical electrophotographic photosensitive body (photosensitive drum) 2 as an image bearing member. Around the photosensitive drum 2 rotated in a direction shown by the arrow, there are arranged a charge roller 4 for uniformly charging a surface of the photosensitive drum 2, and an exposure device 16 for forming an electrostatic latent image on the photosensitive drum 2 by using a light signal 17 emitted from a light source 16 such as a laser and the like. The electrostatic latent image formed on the photosensitive drum 2 is visualized (as a toner image) by a developing apparatus 5 having an yellow developing device 5a containing yellow (Y) color developer, a magenta developing device 5b containing magenta (M) color developer, a cyan developing device 5c containing cyan (C) color developer, and a black developing device 5d containing black (BK) color developer.
On the other hand, a transfer material 14 supplied one by one from a sheet supply cassette is held by a transfer drum (transfer material bearing member) 31 having a gripper 15, and the toner image formed on the photosensitive drum 2 is transferred onto the transfer material at a transfer station.
After the image is transferred to the transfer material 14, the residual toner remaining on the photosensitive drum 2 is removed by a cleaning means (cleaner) 6 for the preparation for the next image formation. In this way, for example, four color toner images are transferred onto the same transfer material in a superposed fashion. Thereafter, the transfer material is separated from the transfer drum 31 by a separation means 3, and then is sent to a fixing device 12, where the transferred toner images are fixed to the transfer material 14.
In the apparatus shown in FIG. 5, as shown in FIG. 6A, the transfer drum 31 comprises both end rings 1a, and a connection member 3lb connecting between these end rings 31a to form a hollow notched drum frame or box. A notched portion or opening of the drum frame is enclosed by a flexible dielectric sheet 1c made of polyethylene telephthalate (PET), polyvinylidene fluoride (PVdF), ethylene propylene fluoride copolymer (FEP), polycarbonate, polyurethane or the like, thereby forming the transfer drum.
Further, as shown in FIG. 5, an attraction roller for electrostatically attracting the transfer material 14 to the flexible sheet 31c is arranged outside the transfer drum 31. In addition, within the transfer drum 31, along a rotational direction of the drum, there are arranged a attraction charger 8 opposed to the attraction roller 7 and adapted to charge the flexible sheet 31c, and a transfer charger 9 disposed at the transfer station. Further, there are also arranged separation electricity removal chargers 10 for removing the electricity from the transfer material absorbed to the flexible sheet 31c, a separation pawl 3 for separating the transfer material 14 from the transfer drum 31, and a sheet electricity removal charger 11 for initializing the potential of the flexible sheet 31c.
Explaining the image formation process of the color image forming apparatus, first of all, a first color electrostatic latent image formed on the photosensitive drum 2 by the exposure light 17 in response to a first color image signal from the exposure device 16 is visualized by the yellow developing device 5a containing the yellow (Y) developer. In a timed relation to this process, the transfer drum 31 holds a tip end (leading end) of the transfer material 14 by the gripper 15, and then the transfer material 14 is pinched between the attraction roller 7 and the transfer drum 31 and at the same time the transfer material 14 is electrostatically absorbed to the surface of the transfer drum by applying the charges from the attraction charger 8 to the back surface of the flexible sheet 31c of the transfer drum 31.
The transfer material 14 held by the transfer drum 31 is conveyed to the image transfer station (opposed to the photosensitive drum 2) by the rotation of the transfer drum, where the image formed on the photosensitive drum 2 is transferred onto the transfer material by the action of the transfer charger 9.
Thereafter, the residual developer remaining on the photosensitive drum 2 is removed by the cleaner 6, and then, a new electrostatic latent image is formed on the photosensitive drum 2 by the exposure device 16 in response to a second color image signal. This electrostatic latent image is developed by the magenta developing device 5b containing the magenta (M) developer corresponding to the second color, thereby obtaining the visualized image. This second color visualized image is transferred onto the transfer material 14 to which the first color visualized image was transferred by the transfer charger 9. Similarly, a third color visualized image is formed on the photosensitive drum 2 by using the cyan (C) developer and the visualized image is transferred onto the transfer material 14 on the transfer drum 31 in a superposed fashion in the same manner as the second color visualized image. Lastly, a fourth color visualized image is formed on the photosensitive drum 2 by using the black (BK) developer and the visualized image is transferred onto the same transfer material 14 on the transfer drum 31 in a superposed fashion in the same manner as the third color visualized image.
The transfer material 14 to which the plural color visualized images were transferred is sent, by the rotation of the transfer drum 31, to the separation electricity removal chargers 10 opposed to each other with the interposition of the flexible sheet 31c. Accordingly, the electrostatic attraction force between the transfer material 14 and the flexible sheet 31c is removed, and then the transfer material 14 is separated from the transfer drum 31 by the separation pawl 3. The separated transfer material 14 is sent to the fixing device 12, where the transferred visualized images are fixed to the transfer material. After the transfer material is separated, the charge on transfer drum 31 is removed by the sheet electricity removal charger 11 to electrically initialize the transfer drum 31. FIG. 9 shows the image formation sequence wherein images are formed continuously with respect to a plurality of transfer materials.
In the above explanation, while an example that the notched transfer drum is used as the transfer drum 31 was explained, as shown in FIG. 6B, it is well known to use a solid transfer drum having no notch, which is constituted by a conductive base or cylindrical drum frame la, an elastic layer lb made of foam material such as urethan foam, CR rubber, EPDM rubber, silicone rubber or the like and coated on the drum frame, and a flexible sheet lc coated on the elastic layer. In this case, the bias voltage is applied to the solid drum 1.
Since the transfer drum 1 of solid drum type can be simplified in its internal construction in comparison with the above-mentioned notched drum 1, the drum can be made cheaper, and, since the flexible sheet lc is supported from inside, it is possible to reduce or eliminate the deformation-and damage of the flexible sheet which is the drawback regarding the notched drum. The color image forming apparatus using such transfer drum of solid drum type has the durability longer than that using the notched transfer drum and can reduce the number of chargers (to be used) which generate ozone. Accordingly, nowadays, color image forming apparatuses using the solid transfer drum have been noticed.
However, in case where the solid drum as shown in FIG. 6B is used as a transfer drum, when the sequence as shown in FIG. 9 is effected, immediately before the transfer material is separated from the transfer drum, it is feared that the toner image(s) on the transfer material is scattered to distort the image. This is the reason why, since the transfer voltage for the first color differs from the transfer voltage for the fourth color, immediately after the fourth color image is transferred, when the transfer voltage for the fourth color is charged to the transfer voltage for the first color, the attraction force for absorbing the toner image to the transfer material is reduced or disappeared before the separation of the transfer material. Further, in order to prevent the distortion of the image when the images are continuously formed on a plurality of transfer materials, if a distance between a first transfer material and a second transfer material is made longer, the number of revolutions of the transfer drum is increased, thereby worsening the productivity and reducing the durability of the apparatus.